The present invention relates to an internal combustion engine comprising an exhaust gas recirculation system, especially an internal combustion engine dedicated to personal vehicles or industrial applications, such as industrial vehicles or machines.
In many countries, environmental regulations impose an upper limit in engine NOx (nitrogen oxide and nitrogen dioxide) emissions, and in future regulations, this limit will again be reduced.
One of the conventional ways of decreasing the level of NOx emissions in internal combustion engines is to recirculate a portion of the exhaust gas back to the engine cylinders. This results in lowering the combustion temperature and oxygen concentration and, as a consequence, limits NOx generation as NOx is generated by oxygen and high temperature. Cooling the exhaust gas recirculation (EGR) gas before reintroduction into the engine further reduces NOx emissions as this allows the introduction into the cylinders of a greater mass of exhaust gas and increases mixture heat capacity.
To meet the current regulations, a typical internal combustion engine can comprise as depicted on FIG. 1:                an air intake line 100 carrying intake air towards at least one engine intake manifold 101 connected to each cylinder 102, said air intake line 100 including an upstream low pressure compressor 103 and a downstream high pressure compressor 104 as well as an heat exchanger 105 (using the coolant of the engine cooling system) located between the high pressure compressor and the intake manifold 101;        an exhaust line 106 having at least one exhaust manifold 107 collecting the exhaust gas formed in each cylinder 102; said exhaust line 106 can include two turbines 108, 109 driven by the exhaust gas flowing from the exhaust manifold 107, each turbine being mechanically connected to one of the compressors 103, 104;        an exhaust gas recirculation (EGR) line 110 whose inlet is connected to the exhaust line 106 and whose outlet comes out in an EGR mixer 111 connected to the air intake line 100, before the intake manifold 101 and after the intake cooler 105, whereby part of the exhaust gas is mixed with intake air and then reintroduced into the engine cylinders 102.        
A control valve 113 also referred to as EGR valve regulates the flow of exhaust gas rerouted from the exhaust manifold 107 into the intake manifold 101.
In such a known engine, the EGR gas is cooled before entering the cylinders 102 by means of an EGR cooler 112 located in the EGR line 110; this cooler 112 is usually an air/water heat exchanger using the coolant of the engine cooling system.
Consequently, the heat transferred from the hot exhaust gas to the coolant can be significant, which can be detrimental to the cooling capacity of the vehicle cooling system.
Tighter NOx emission regulations will therefore result in vehicle cooling systems needing more cooling power. Therefore, coolant pump design could be problematic, and fuel consumption could be significantly increased as cooling fans may have to be engaged more often to meet the extra cooling need.
Another technical issue that has to be taken into account is the engine pressure differential. In order words, for the EGR gas to be able to flow from the exhaust manifold to the intake manifold, the engine pressure differential (which is the difference between exhaust pressure and intake pressure, i.e. dP=Pexhaust−Pintake) must be positive and significant enough. However, under specific engine operating conditions, exhaust backpressure can be lower than intake pressure (i.e. dP is negative) or not high enough. This generally occurs at low engine speeds or low loads. Consequently, under these engine operating conditions, no or too little EGR gas is rerouted into the intake manifold, and therefore NOx emissions cannot be reduced under the level imposed by regulations. This positive exhaust to intake pressure difference will also affect engine efficiency and increase fuel consumption.
WO 01/14707 tackles the problem of EGR gas cooling and of engine cooling system overload. Under the teaching of this document, the EGR cooler has to be oversized, as EGR gas flowing from this EGR cooler goes through a compressor and then is reintroduced into the intake manifold without further cooling.
Moreover, since a single turbine is provided on the exhaust line to drive two compressors, namely an intake air compressor and an EGR gas compressor, the engine thermodynamic efficiency is not optimized.
Another engine provided with an EGR system is described in WO 98/35153. According to this document, EGR gas flows through a radiator, and then through a compressor before it is reintroduced into the intake manifold. Consequently, the EGR gas temperature increase taking place in the EGR compressor can be compensated by a prior temperature reduction in the radiator, in which the EGR gas cooling is achieved by air flow.
While this arrangement is profitable since it does not entail an overload of the engine cooling system, on the other hand it has several other drawbacks.
In particular, in order to achieve a sufficient decrease in EGR gas temperature, the radiator must be large enough, and located in a sufficiently open space to allow air to flow around it. However, a vehicle has a complex structure which includes a large number of components (engine, cooling system, suspension system, transmission system, hydraulic system etc.) which are very tightly arranged so as to minimize the overall size of the vehicle. The consequence is that the space dedicated to accommodate the radiator can be severely limited.
More generally, the engine arrangement described in WO 98/35153 involves many conduits, and consequently many associated components such as valves, etc. In addition to being complex, such a structure also lacks compactness.
JP-2001073884 discloses a turbocharged engine arrangement where the turbocharger is equipped with a waste gate to relieve the turbocharger when too much exhaust gases flow out of the engine. The waste gate ejects excess gases in a line which is connected to the intake manifold, so that these gases are re-circulated. This line, which is branched-off the waste-gate of the turbocharger, therefore forms a kind of EGR line, but it has the disadvantage that the EGR flow is directly linked to the amount of gas discharged from the waste-gate. Therefore, it is not possible to control independently the flow of gases through the turbine of the turbocharger and the flow of EGR. Therefore, for certain operating conditions, it is not possible to fully optimize the engine's operation.
It therefore appears that there is room for improvement in the exhaust gas recirculation system in internal combustion engines.
It is an desirable to provide an improved internal combustion engine equipped with an exhaust gas recirculation system, which can overcome the drawbacks encountered in current engines.
It is desirable to provide an engine where EGR gas can be cooled enough without overloading the engine cooling system.
It is also desirable to provide an engine with a better thermodynamic efficiency.
Thus, the present invention provides, according to an aspect thereof, an internal combustion engine that comprises a plurality of cylinders, an air intake line capable of carrying intake air towards an engine intake manifold and an exhaust line capable of collecting exhaust gas from an exhaust manifold. The internal combustion engine also comprises an EGR line capable of rerouting a part of the exhaust gas from the exhaust line towards the air intake line and at least a first turbocharger comprising a first turbine driven by the exhaust gas flowing towards the atmosphere, mechanically linked to a first compressor located on the air intake line. The internal combustion engine further comprises a turbine located on the EGR line driven by the EGR gas flowing in the EGR line. The EGR turbine is of the variable geometry type.
With this arrangement, EGR gas flowing from the exhaust manifold goes through the EGR turbine prior to entering the EGR cooler. Due to the pressure reduction occurring in the turbine, the EGR gas temperature is lowered, for example by as much as 100° C. Consequently, the EGR gas temperature at the EGR cooler inlet is lower than in the prior art engines. This makes it possible to reduce the load on the engine cooling system and to obtain a lower EGR gas temperature at the intake manifold inlet, which means an even more reduced NOx level in the exhaust gas. Moreover, to achieve this goal, the invention does not require a large and cumbersome radiator. It has to be noticed that, even if an air to water heat exchanger is still present in the engine to cool EGR gas, the heat rejection to the engine cooling system is however lowered thanks to the invention, since the expansion through the EGR turbine makes it possible to save a significant part of the vehicle cooling capacity. The fact that the EGR turbine is of the variable geometry type allows an optimal control of the EGR gas temperature and pressure reductions through the EGR turbine. Indeed, the flow of EGR gas can thus be regulated by varying the geometry of the turbine. Indeed, a direct effect of varying the turbine geometry is to vary the pressure drop of the flow of EGR gases through the EGR turbine, which influences the flow of gases in the EGR circuit compared to flow of gas in the main exhaust line.
The EGR turbine being located on the EGR line is solely driven by EGR gas rerouted into the intake manifold, the EGR turbine is not driven by exhaust gas flowing towards the atmosphere. In other words, the EGR turbine is an EGR dedicated turbine, arranged in parallel with the first turbine of the first turbocharger; all the flow passing through the EGR turbine is fed to the air intake line.
The EGR turbine being of the variable geometry type, no additional flow regulating valve needs to be provided in the EGR line upstream of the EGR turbine. This not only simplifies the construction and the control of the engine arrangement, but it also allows to optimize both the flow of EGR and the EGR turbine operation in a combined way, achieving optimum efficiency of the system.
According to a preferred implementation of the invention, the internal combustion engine further comprises an energy recovering means linked to the EGR turbine and capable of recovering the energy provided by the EGR turbine.
This important arrangement of the engine makes it possible to recover the energy produced by the EGR turbine in an appropriate energy recovering means, which can directly use this energy or store it for future use. Consequently, thanks to this implementation of the invention, on top of a better EGR temperature decrease, a better engine thermodynamic efficiency can be achieved.
Preferably, the EGR line outlet is connected to the air intake line upstream from at least one compressor.
In that way, the invention makes it possible to manage engine air pressure differential dP, since the compressor forces EGR gas to flow towards the intake manifold even at engine operating conditions when dP would be opposite or favourable but too low. Therefore, the engine back pressure is significantly limited: EGR gas will naturally flow from a high pressure source to a low pressure source and fuel consumption can be improved. Because there always exists an EGR gas recirculation, engine NOx emissions can be effectively reduced under the imposed level, whatever the engine operating conditions.
According to a first embodiment of the invention, the energy recovering means is a second compressor mechanically connected to the EGR turbine and capable of compressing gas flowing from the first compressor outlet towards the intake manifold.
In this embodiment, the engine comprises two turbochargers whose turbine, driven by exhaust gas or EGR gas, provides energy for compressing intake air or a mix of intake air and EGR gas. The gas flowing in the air intake line towards the intake manifold can then pass through a two-stage turbocharger. This arrangement may be implemented in order to provide an intake pressure which is high enough to create a favourable engine pressure differential.
The second compressor can be located on the air intake line, downstream from the first compressor. When needed, the air intake line may further comprise an additional compressor located downstream from the first compressor. This additional compressor is preferably situated between the first and the second compressors, and may be part of an additional turbocharger, the turbine of which being located on the exhaust line upstream from the first turbine.
Alternatively, the second compressor is arranged in parallel with an additional compressor located on the air intake line downstream from the first compressor.
According to a second embodiment of the invention, the energy recovering means may be an energy storage component (such as a battery), a crankshaft mechanically or electrically connected to the EGR turbine, or an electrical device connected to the EGR turbine (such as an electric motor or an alternator).
This second embodiment can be implemented when no compressor linked to the EGR turbine is required on the air intake line to obtain a satisfactory engine pressure differential. Consequently, the energy provided by the EGR turbine can either be directly used by another energy recovering means, or stored in an energy recovering means for a future use or for a use by another device located farther.
According to a third embodiment of the invention, the variable geometry EGR turbine is linked to a shaft of the at least first turbocharger said shaft connecting the first turbine and the first compressor. In other words, this embodiment of the invention incorporates a single compressor which is driven by two turbines namely an EGR turbine driven by EGR gas and a turbocharger turbine driven by engine exhaust gas. In this embodiment of the invention, the energy that is recovered on the EGR turbine is added to the energy that is recovered by the turbine of the engine turbocharger. The fact that at least the EGR turbine is of the variable geometry type allows to operate both turbines at optimum operating conditions, despite the fact that their rotation speed is not independent while operating under possibly different and varying gas flows.
In order to increase the intake pressure, the air intake line may further comprise an additional compressor located downstream from the first compressor. This additional compressor can be for example part of an additional turbocharger the turbine of which is driven by exhaust gas.
The EGR line outlet can be connected to the air intake line upstream from the only compressor or from the compressor located most upstream. This would result in a better mixing of EGR gas and intake air, and thus a better cooling of EGR gas since they may flow through more coolers and compressors. Additionally, if several compressors are provided, EGR gas pressure would also be higher, which favours NOx emission reduction.
Alternatively, the EGR line outlet can be connected to the air intake line downstream from the compressor located most upstream and upstream from at least one other compressor. With this disposition, the EGR pipes may be shorter, the engine being less expensive and more compact.
In an advantageous way, the exhaust manifold is arranged in two parts, each connected to a corresponding EGR pipe, the two EGR pipes meeting upstream from the first turbine. This prevents the EGR turbine from getting energy from only one part of the exhaust manifold, receiving only a few exhaust pulses, which would lead to an irregular driving of said turbine and to a poor efficiency.
Besides, the air intake line can further comprise at least one cooler located downstream from the EGR line outlet. This makes it possible to lower the EGR gas temperature in at least one cooler before it is reintroduced into the intake manifold.
For example, the air intake line can include:                an intake cooler located downstream from the compressor situated most downstream, and upstream from the intake manifold;        and/or at least one intake cooler located between two compressors when at least two compressors are present.        
These and other advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as non-limiting examples, embodiments of an engine according to the invention.